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Do pilots adjust the aircraft's flight path to allow for the curvature of the Earth?
If an airliner is flying at approximately 500-600mph, it seems to me that there would need to be a significant adjustment for altitude so as to not fly off into space.
Wikipedia says that there's an 8 inch drop for every mile because of the Earth's curvature. But, I've never heard of any airliner adjusting for the curvature. Also, shouldn't aircraft have to adjust somehow for the Earth's rotation because it varies depending on the latitude?

Why don't ships or cars "fly off into space"? Speed doesn't matter (well, not at such slow speeds anyway :)), but one thing ships and airplanes and cars have in common does.

@Pondlife A small quibble: Wikipedia does not say "that there's an 8 inch drop for every mile"; it merely quotes Samuel Rowbotham's method for calculating the Earth's curvature, which uses that formula without justification. It doesn't claim it as fact or say anything about whether it is correct or accurate.

Maybe the board computer take into account this .

https://fr.quora.com/Un-pilote-davion-volant-manuellement-doit-il-ajuster-la-trajectoire-ou-le-cap-de-son-avion-%C3%A0-cause-de-la-courbure-de-la-Terre-et-si-oui-%C3%A0-quelle-fr%C3%A9quence (french link sorry )

Aircraft altitude is measured (inferred) by atmospheric pressure. The aircraft is usually flown at an altitude that maintains constant ambient pressure (by pilot or autopilot, as the case may be). Changes in local barometric pressure (provided by air traffic control) are used to recalibrate the aircraft altimeter.  As long as the aircraft is flown at a constant ambient pressure (hence constant altitude), it will be following the earth's curvature (as the atmosphere is attached to the spherical earth and has same properties at same distance from the center, in an ideal case) as the altitude is measured from the surface, which is curved, and not a plane.

There is no adjustment needed as the aircraft will naturally follow the curvature of the earth without any input from the pilot. This is because the aircraft flies through the atmosphere which also follows the curvature of the earth.

And the gravitational force works at right angles with the surface of the earth too.

@SMSvonderTann you might find that an "almost" is missing in your sentence (http://earthscience.stackexchange.com/q/7520)

@SMSvonderTann if that were true, you'd be able to walk up cliffs.

@Federico I realised that after I put the comment up.

@Federico, while the gravitational force does indeed act “almost” perpendicular to the Earth surface, it is irrelevant, because the force we are concerned with is the weight and that works exactly perpendicular to the geoid.

@SMSvonderTann, actually, you didn't miss “almost”, you used inexact term. The force that matters is weight, not gravity (and weight is exactly perpendicular to the surface defining sea level, the geoid).

@JanHudec Sorry about that. I had just woken up.

@SMSvonderTann, almost everybody uses “gravity” and actually mean “weight”. Does not really make that much difference in most cases.

@JanHudec Small variations in the density and the shape of the earth result in small variations in the force resulting from gravity. Only a truly perfect, uniformly dense sphere would result in perfectly perpendicular force. Anyway, your claim that "only the weight matters" is nonsensical. Even if you do mean something different from the gravitational force (although I can't fathom what that would be), you can construct a completely correct (within error of Newtonian physics) mathematical model of the motion that uses the gravitational force.

@jpmc26, the Geoid, or “sea level” is defined as equipotential surface of weight and, since the weight field is smooth, the weight force is always perpendicular to it, exactly. The Earth also does actually assume that shape, except for terrain features, because rock is soft enough to behave as liquid (and most of it is molten inside Earth anyway).

@jpmc26, ‘weight’ means the total inertial force in a reference frame of Earth that is sum of gravity proper and centrifugal force due to rotation of Earth. Sometimes the term ‘gravity’ is used for just the gravity proper and sometimes for the sum, so I am avoiding it. The weight is the only thing that matters because gravity is indistinguishable from inertial force due to acceleration of reference frame.

Vertical datum / geoid related --- and --- this.

@JanHudec You can't say that "weight" is the gravitational force plus the fictitious forces and then say that the constituent forces don't matter. That's a nonsense argument arising from poor semantics; you can construct an equivalent model without summing them. It appears you're using the ISO convention for the word "weight," but this isn't the only convention. Also, your definition of the geoid makes no sense. There will be many equipotential surfaces in a potential energy field. Last, of course the gradient is perpendicular to such a surface; this arises from how you defined things.

@jpmc26, I am aware the terminology is not used consistently. So which one would you use? And why would you prefer it over ISO one; after all, purpose of ISO standard is to make the terminology more consistent. Also keep in mind that while you can construct a model where you won't name the sum, it is still that sum that acts on the objects (as long as we stick to reference frame of the Earth), so not naming it does not seem exactly practical.

"There is no adjustment needed as the aircraft will naturally follow the curvature of the earth without any input from the pilot." No it wont. Without input from the pilot or autopilot, the aircraft will naturally smash into the earth and leave a massive, flaming crater. Continual adjustments are made to maintain the distance from the surface of the earth, i.e altitude. As long as a consistent altitude is maintained, the aircraft will follow the curvature of the earth

@JanHudec: Strange that nobody mentioned it, but unless you fly exactly over the equator, the centrifugal force of the earth's rotation will pull the combined acceleration vector which we feel and call gravitation away from the center of the earth's mass. The effect is small but measurable and would justify the "almost" that Federico found missing. To be fair, I am not happy with any of the answers and comments here - why does nobody explain why the atmosphere is curved? This would get to the root of it.

@PeterKämpf, away from centre of mass yes, but not oblique to the surface. The reference ellipsoid takes the centrifugal force into account, so the result is orthogonal to its surface (and the geoid also adds the variations in gravity due to inhomogeneities).

There isn't an adjustment for altitude. An aircraft flying level at a given altitude and trimmed for level flight will stay at that altitude. That means the flight path will have a gentle nose-down curve (looking from far away from the earth) as the direction of down (towards the centre of the earth) changes.
Think about the gravitational potential energy of the aircraft. To climb (which is actually flying in a straight line when you consider the curvature of the earth), the aircraft has to gain energy. In a level flight attitude, it doesn't gain any energy, so it will stay at the same altitude. A path that doesn't gain or lose altitude is an ellipse that goes around the earth.
Another way of thinking about it is to consider how "down" changes as the aircraft travels. The weight of the aircraft always acts towards the centre of the earth, and is matched (in level flight) by the lift of the wings. Imagine if you had a model aircraft suspended on a piece of string, dangled from your hand. If you hold the string and carry the model a quarter of the way around the earth, the bottom of the model will still point down (towards the centre of the earth). The model has rotated 90 degrees, without you having to rotate it by hand.
When you trim for level flight, you do so by finding the pitch attitude where your speed and altitude remain constant (or at least stable: atmospheric conditions might make them fluctuate a lot). That attitude might be a touch more nose-down than it would be if the earth were flat, but it's imperceptible.

"downward" is a little abstract in this usage.  The airplane will stay level relative to the earth.   Relative to some fixed point beyond earth's reference (a distance satellite?) the airplane will change orientation, but there will no longer be a concept of "up" and "down" from that perspective.

@abelenky I think I've found what was unclear and tried to make it a bit clearer. Do you think it's better now?

@DanHulme Hey Dan.  Why an ellipse?  For practical purposes, we assume that the Earth is a perfect sphere.  Surely the path would therefore be a circle?  I do like the model aeroplane analogy.

@Simon Earth is 'fatter' at the equator than across the poles. The difference in diameter is about 26.5 miles. That is, you're 13 miles closer to the center of Earth if standing at sea-level at a pole vs. standing at a sea-level point on the equator.

Gravity doesn't pull you around like a plane on a string. If you can fly level then gravity can't stop you already, if you fly off the curve gravity would be weaker not stronger, it would be a tangent off angle and you should be rising in altitude.

@A.Danischewski It's not about the gravity changing your direction, it's that the aircraft is balanced so that it's in a particular attitude with respect to the direction of gravity. When you take the model (or the real aircraft) to a different place, the balance always keeps it perpendicular to its weight. It's a huge simplification (because the balance on an aircraft is aerodynamic), but it's about the rotation of the weight, not about how the magnitude of the weight would change with altitude.

@Simon If you want to assume the earth is a sphere, feel free. But since all circles are also ellipses, you can read the answer in the same way :-)

@ Dan Hulme The physics isn't going to support your idea, gravity doesn't magically and seamlessly spin an airplane around with it like it's got a string attached from the center of it to the center of the Earth. Draw a circle and some arrows pointing from outside toward the center, then draw your aeroplane heading off the side of it -- you can see that the gravity vectors will now instead of pulling directly down, are swept back now partially pulling the aircraft backward.

Furthermore, try spinning a ball and put a little model plane over it flying in the direction of the spin note the relative ground speed. Then turn the little plane around, with same ball spin and the same relative air speed of the plane and the ground speed jumps up quite a bit.

@A.Danischewski I'm sorry if that last analogy is confusing: it seems like you haven't understood what I'm trying to say. The string in the analogy is not about gravity, it's just a way of seeing that the model is balanced and level. As you say, when the aircraft flies, gravity now pulls it backwards, so the balance will change (viewed from outside), keeping the fore-and-aft line perpendicular to the line of action of the weight.

Hi Dan, +1 for mentioning gravitational potential energy. I found this missing from all other answers, but think it is essential for a good explanation. Kevin's answer comes close, though.

This is more of a physics question rather than an aviation question. While other answers have addressed the question from the aerodynamics point of view, let me try answering it from a physics perspective: frame of reference.
Frame: where are you going?
How do you know an object is moving? The answer is you don't - there is nothing like a "fixed absolute coordinate in space". Speed is measured by referencing another object. If you think of it, every time we mention an object's speed, we always mean it's speed relative to something. The car's speed is 50mph relative to the ground below it, although we seldom say it explicitly in our daily conversations.
Curvature of the Earth
Most of our daily experiences of motion are related to one of the four fundamental forces of nature: gravity. Gravity is spherical - if we pick a random point on the planet, the gravitational force should be the same as any other random point (let's assume the Earth is a perfect sphere for now).
Consider me walking on the surface of the Earth. I would be unaware that the Earth is spinning without external reference points like stars and the sun. If the Earth stands still, I would walk just the same, because when I walk I am interacting with the ground below me, which is part of the Earth; I am not interacting with Saturn or any other planet.
How do planes fly?
Well, it happens that planes fly by interacting with the atmosphere. The atmosphere is affected by the Earth's gravity like the ground - the ground+atmosphere spins round and around together. Airplanes are not interacting with other stars or planets or satellites in any way - its engines produce thrust against the atmosphere (which moves together with the ground).
You chose your frame of reference as an arbitrary point in space, which is why it leads to incorrect conclusions. If you are travelling on a train and you want to move around, you don't have to factor in how fast the train is moving - you are interacting with the train, not the rail which the train travels on. Otherwise, as the train moves forward following the curvature of the Earth, you will rise higher and higher in the cabin, eventually hitting the ceiling. You are following gravity, which is spherical. Same for planes.

"If the Earth stands still, I would walk just the same." Well, excepting for the effects of the earthquakes or similarly-destructive geological events that would result from that. :) The extra 13 miles of radius at the equator would suddenly become a problem. The weather could get interesting, as well.

@reirab - The first chapter of the What If? book (which is widely excerpted online) discusses exactly that.

@Bobson Oh, I was thinking if the atmosphere stopped, too. Yeah, leaving the atmosphere as is while stopping the surface would be even worse. Also, xkcd is wonderful.

Be careful - "engines produce thrust against the atmosphere" - no they don't.  They produce thrust by accelerating an expanding mass of air backwards from the exhaust or by driving a prop which accelerates air backwards.  Newton's Third Law.  If you could somehow supply air to the intake of a jet engine, it would work perfectly well in a vacuum.  Of course, a prop wouldn't work since it would have no air to accelerate backwards.

@Simon well, we usually don't carry liquid oxygen and inject it into an jet engine (-:

Sure, I was just pointing out to be careful.  There are many anti-science types who claim that spacecraft are impossible since there is "no atmosphere" to thrust against.  I would hate to see anything on here get linked to such nonsense.

Is a plane fast enough to get to space? Well, unfortunately for space exploration, no, it isn’t. Let’s check with a few calculation.
Assumptions and definitions
First, let’s make an assumption: our plane P is flying in the void: there is no air slowing it down. Of course, this assumption is very wrong, but we only care about speed here. Air friction only slows P down. So, if P doesn’t go into space without an atmosphere, it won’t either if we add an atmosphere.
Let’s consider a reference object O orbiting the Earth at the same altitude r as our plane. Of course, we can neglect the mass of P and O, which is several orders of magnitude lesser than the mass of the Earth.
Let vO be the orbital speed of O and vP be the speed of P. If vP > vO, then P will spiral away from Earth and end up in space. If, on the other hand, vP < vO, then P will spiral down until it crashes.
Computing vO
Because O is orbiting on a circular trajectory, vO and r are correlated by the following formula:

vO = √(GM/r)

Where G is the gravitational constant and M the mass of the object we’re orbiting around.
In our case, we are orbiting around the Earth and

GM = 3.99×1014 m3s-2

By using the SI units and approximating 3.99 with 4, we end up with the following relation:

vO = 2×107 r-1/2

The radius of the Earth is included in r, so we must add 6 371 km to go from altitude to r. 6 371 km is actually two order of magnitude higher than any altitude you could fly at, so let’s round it to 6,400 km and be happy with it.
So, in order to orbit at 6 400 km = 6 400 000 m from the center of Earth, we would need to go at

vO = 2×107 r-1/2 = 2×107 (6 400 000)-1/2 m/s = 2×107/2530 m/s = 7.9×103 m/s = 1.8×104 mph

That’s the speed relative to the center of the Earth, not to its surface.
So, does our poorly modeled plane fly?
Our plane P, is flying at 600 mph relative to the surface. If we suppose we’re flying above the equator, we can reach a speed of 1600 mph relative to the center; that still is an order of magnitude below the speed we need to reach to orbit at such a low altitude.

vP << vO

Our plane would crash. Fast.

Thank goodness, planes are safer than that. But how come?
Well, as we’ve just seen, the speed of the plane is not what keeps it flying. But we made a very strong assumption in our opening section: we neglected the atmosphere.
Air certainly slow our plane down. But we’ve also built it so that, when moving through air, our plane tends to go up thanks to lift.
So we have two opposing effects, here: the plane isn’t going fast enough to escape the Earth attraction; but the lift pulls up the plane hard enough that it keeps flying. But lift depends, among other things, on the density of air around the plane: the higher the planes fly, the weaker lift is. So, all in all, the plane flies in a layer of air at the same pressure, which in turn follows approximately the surface of the Earth.

"several orders of magnitude lesser"  understatement of the year ;)

@Simon Haha, yeah, it's approximately 19 orders of magnitude... for an A380 at MTOW.

For your information; You somewhere claimed that an airplane orbits 6400 m from the center of Earth. That should have been 6400 km of course, so I suggested an edit to fix it.

Orbiting is not necessary to be "in space" or to "leave" Earth attraction. In theory, you can go to Mars vertically, climbing 1 mm per day if you have the fuel required for that. Orbiting speed is useful only for unpowered spacecraft (this is always the case in practical).

@wythagoras Ah! I knew I screwed up somewhere when I went from thinking “it’s nearly a perfect square, too bad there’s an extra 10 factor” to “wait, it is a perfect square”.

This gives the real answer: the plane doesn't fly off into space because 600mph is not fast enough to send it flying off into space.

The posted answers are good.
A tangent (pun intended) answer: if adjusting for curvature of earth was required, it infers that you could accidentally fly into space.
If you could accidentally fixed-wing fly into space, we wouldn't need sophisticated and powerful rockets.

And anyway you can't fly into space, as flying requires air to create lift. No air, no lift, no climb.

Cute, but this argument doesn't actually work. The consequence of not adjusting for the Earth's curvature could be something other than accidentally flying off into space. E.g., at high enough altitude, your wings won't generate enough lift and your jet engines won't have enough oxygen to burn fuel with. Maybe you'd need too much fuel to get to that altitude anyway. Maybe the low pressure would burst the hull. It's possible that planes adjust for curvature to avoid these catastrophes, not to avoid flying into space. (Of course, they don't, but the question is premised on now knowing that.)

It's no different than driving.  If you drive absolutely straight you'll eventually leave the road.  The road lines are meant to be followed.
So what is equivalent to the road lines in flight?  Air pressure.  The air pressure reduces the further you are from the surface.  Pilots and autopilots follow the air pressure gradient, trying to keep the plane at a set air pressure.  This pressure is used rather than GPS or "straight flight" because it's one of the many factors that affects flight efficiency - speed vs air resistance vs load bearing capacity of the aircraft.  They are flying, or attempting to fly, in a pressure range that is going to cost the least to accomplish the various goals of the airline.
The air pressure varies according to many factors, but the main factor is height from sea level, and so by flying inside a specific range of pressures, they maintain a reasonably constant height from sea level.  Since "sea level" pressure is curved along with the earth, then what you find is that they automatically follow the curvature of the earth.  In other words, the planes are flying slightly down all the time.

@mins correct, air pressure varies according to many factors.

Planes don't stay perfectly level!
That's the long and short of it. As these 600 mph machines go that 1 mile forward, both gravity and air pressure force the plane down those 8 inches. More, actually; these massive metal monstrosities have to put up a fight to be able stay up in the air! That's what the elevators, rudders, engines, airfoils, etc. are all there for.
Of course, if it were going sufficiently fast (at least 25,020 mph), it would overpower gravity, leave the atmosphere, and enter orbit. I hope it has engines that don't rely on air, and a body made of reinforced carbon-carbon!

It's not necessary to reach escape velocity to enter orbit.  In fact, it's necessary to have a smaller velocity, because if you're traveling at escape velocity, the force pulling you back toward the earth will be too small to make you reverse direction.

@phoog somehow the comments on this answer were wiped. In them, I said that the original value I'd written was low-earth orbit velocity (17,400 mph), but just to be sure the plane would leave the atmosphere and enter orbit, I bumped that up to escape velocity.

I think there are a lot of good answers in regards to the curvature of the Earth, but I didn't see anyone addressing the spin of the Earth.
The atmosphere is naturally dragged along with the surface of the planet. Wind speed is measured relative to a fixed location on the surface. Therefore differences between the movement and the Earth's surface are reflected as wind. Pilots do adjust for the effect of wind on the flight path.
If the pilot is using basic navigation techniques, he would calculate the heading he had to fly with the predicted crosswind to end up at his destination. If there is any crosswind component, the flown heading will be different than the direct heading on the ground or great circle heading.
With navigation equipment these adjustments can be done in real time. for example if you want to fly a specific GPS heading, but keep drifting to the right you would turn to a more left heading to regain and keep the desired heading.

There have been good answers, but all seem to miss one aspect.
The aircraft does not maintain a constant distance to the surface of the earth. It simply maintains a level, where the atmospheric pressure is at a set target.
To clarify:
Autopilot (or the pilot) wants to maintain a contstant reading on the altimeter. This reading is merely a calibrated difference to a reference level, which normally is set to 1013,25hPa (or 29,92inHg) on cruise flight. This means that maintaining an altitude of about 30000ft, you want to maintain a pressure level of 300hPa. I say about 30000ft, because the true altitude of this level varies greatly, affected by airmass temperature and air pressure.
Jan Hudec was close to this on his answer, though I'd like to make a few corrections. First of all, the levels aren't isobars. Isobars is by definition "an imaginary line or a line on a map or chart connecting or marking places of equal barometric pressure" source. Second of all, there's no need to over-complicate things: the (auto)pilot monitors the altimeter, and does the necessary corrections. It would do so if the aircraft was out of trim also.

That's true, but the reason I didn't mention it (I can't speak for anyone else) is that it doesn't really help answer the question. It just makes things a little harder to understand. Some student pilots get quite stuck with the idea that flight levels aren't levels of constant altitude (neither by QNH nor by true altitude), so I thought it would be easier to simplify and assume altitude. After all, there's no reason you couldn't fly at constant true altitude.

He obviously meant isobaric surface.

Lots of good answers, but nobody  has addressed the simple mathematical aspect of the question.
Assuming the “8 inches per mile” is accurate, that means you’d have to “nose down” to a pitch of -1” per 7920” flown, or a pitch of -0.007 degrees. That’s pretty much level flight. The pilot nor autopilot would even be able to discern the difference. You can’t even eyeball 1 degree let alone 7/1000ths of a degree.

An important flight instrument is the Attitude Indicator, which has an artificial horizon line.  This instrument tells the pilot if the wings are level and whether the nose is pointing above or below the horizon.
Rather than never taking the curvature of the earth into account, when a pilot is using this instrument to keep his flight level, he is actually constantly adjusting for the curvature of the earth.
EDIT: The following link is to a question on how attitude indicators are kept accurate.  A particularly acrobatic flight might cause it to tumble and become useless until it is reset.  However it easily maintains accuracy during normal flights, including sustained turns.
How are attitude indicators kept accurate
Short version: Yes, a gyroscope would have problems with earth curvature, but another part of this instrument constantly maintains a "local down" for the instrument.

I'm not for sure, but over long flights, doesn't the gyro in an attitude indicator have to be adjusted, in part because it does not follow the curvature of the earth naturally? I thought there was question about this a few weeks back, but can't find it right now.

This seems to be the perfect counter-example, if we exclude its drift, a gyroscope keeps a fixed orientation "in space", so following its plane of rotation will change the aircraft altitude, as the OP assumed.

@OSUZorba Yes, there is an erection mechanism that keeps it aligned with gravity (and that may give you false readings for instance if you keep flying in circles for a long time). If the artificial horizon was a perfect inertial platform, it would indicate inverted flight after a sufficiently long time flying along the Earth's curvature!

I added details on the auto-correction of this instrument.

Your question is basically "What makes the flight change its direction continuously?"
Let's consider the case of a plane flying overhead right now. If we are told that the plane is in level flight, the vertical, downward pull of gravity is equal to the vertical, upward lift. Further both forces are perpendicular to the fuselage and cancel out.
Imagine what happens a very very short time later when the plane has moved a very small distance ahead in a straight line. Since the plane has moved in a straight line, the lift is still pointing upwards and perpendicular to the the fuselage, but gravity is pointing towards the center of the Earth. So now only a part of the lift is countering the gravity, and there is a resultant gravitational force that will pull the aircraft down towards the center of the Earth.
This is very similar to how you can tie a stone to a rope and whirl it. The stone will go around in circles. The force of the string (tension) keeps the direction of the stone changing continuously without affecting the speed.

I'm missing one thing mentioned in the answers. The point is: They say that the gravitational force and the lift force's vertical component have to be equal to make the plane fly levelled. This is not entirely true. The plane is also turning, which needs some force. Let's see how much it is, for a plane at $10\,{\rm km}$ flying $900\,\rm{km\,h^{-1}}$:

Gravity: $-mg = -m\cdot9.8\,{\rm m\, s^{-2}}$
Force needed for turning: $-mv^2/r = -m\cdot (250\,{\rm m\,s^{-1})^2} / 6388000\,{\rm m}=-m\cdot0.00978\,{\rm m\, s^{-2}}$
($m$ is the weight, $g$ is the gravitational acceleration, $v$ is the speed and $r$ is the turning radius)

So the force needed for turning is $1/1000$ of the gravitational force. This is negligible in the planes power and other settings, and also much smaller than any irregularities such as varying air density, wind speed etc.
Conclusions: the adjustment for "turning down" is so little that it's negligible.

and your point is? are you answering the question, commenting or asking something else altogether?

My point is, no, they do not adjust for it since the necessary adjustment is negligible.

I am not really qualified to answer, but did so on account of the many answers implying that some form of pilot control is necessary to keep an airplane following the curvature of the earth. Input from a real aerodynamicist would be appreciated.
Several answers state that when an airplane is flown to a constant altitude, it implicitly follows that curve. True enough, but the question asks whether pilot control is necessary. Are there physical constraints relevant to the question?
If an airplane were to follow a tangent, rather than the curvature of the earth, it would experience at least two effects: the direction of the earth's gravitational field would change, causing its weight vector to rotate rearwards, and the local air density would decrease as the aircraft's distance to the earth increases.
As the weight vector rotates  backwards, it increasingly retards the airplane, and if it is longitudinally stable, it will, as a result, tend to pitch downwards. The effect of decreasing density seems more complicated, on account of its effects on power as well as aerodynamic forces, but there seems to be a general tendency for a trimmed, longitudinally stable airplane to be stable in density altitude - see What does it mean for a plane to be aerodynamically stable?. It is certainly true that every airplane has a maximum achievable altitude, determined by physics.
Both of these effects (longitudinal stability with respect to the local gravity field, and stability in density altitude) will tend to guide the airplane to follow the curvature of the Earth without pilot input.

For reasons that are explained in several other answers, there is no need
for an aircraft pilot to make any special calculations or adjustments of
the controls in order to "drop" the aircraft eight inches per mile
and thereby follow the Earth's curvature.
Instead what pilots actually do is to keep the aircraft at a constant
pressure altitude, which (approximately) follows the curvature of the earth.
The path followed by an aircraft in this fashion actually can rise or
fall quite a bit due to weather-induced variations in pressure, so it's
not guaranteed to drop 8 inches each and every time the aircraft flies a
mile, but it will follow the curve of the Earth much more closely than it
follows a straight line over any long-distance flight.
So the answer to the first part of your question is, yes, pilots do
adjust the aircraft's flight path to allow for the curvature of the Earth,
and this is how they do it.
There is no explicit "adjustment for curvature" term in the pilots'
(or autopilots') calculations, however, because they
keep the aircraft on the desired (curved) path by observing the
pressure altitude, much like the
way you would keep an automobile in its lane on a highway
by observing the lane markers,
which does not require you to know anything about the radius of
curvature of each curve the highway engineers laid out.
If the curve is gentle enough you might not even notice that
the road is curved, yet you would still be following it.
A notable difference between highway lanes and the altitude of an aircraft,
however, is that the aircraft does not have the ability to fly above
a certain altitude at a certain setting of the controls, due to the
low density of air at higher altitudes--so at higher altitudes the
aircraft will tend to descend--whereas at much lower altitudes
the aircraft's engines will generate more than enough power to keep the
aircraft aloft and so the aircraft will tend to climb.
The physical properties of the atmosphere and the aircraft therefore
tend to deflect the aircraft along a path that (approximately)
follows the curve of the Earth, and they absolutely prevent the aircraft
from continuing along a straight tangent line into outer space.
A large part (one might say the most important part) of how pilots make the aircraft stay at a given pressure altitude is by adjusting the "trim" of the aircraft (including throttle) in order to make it tend to fly at the
desired altitude.
As for the Earth's rotation, since the atmosphere generally rotates along with the rest of the Earth, and aircraft fly in the atmosphere, the aircraft do not have to fly faster or slower to overcome the Earth's rotation, any
more than you have to be able to run 500 mph in order to make your way
from the aft restroom forward to your seat.
(Some bits of the atmosphere do sometimes go a little faster than the Earth's rotation, sometimes a little slower, depending on which way the wind is blowing at each place; pilots do to account for the wind in
order to fly the desired path over the ground and to know how long it
will take to arrive.)
On the other hand, pilots very much do have to take the curvature of the earth into account when planning the lateral direction in which to fly.
For example, to fly from New York to London, the shortest path (fewest miles over the ground or water) heads out of New York on a course of about 51.4 degrees. The return flight starts out toward New York on a course of about 288 degrees.
Similarly, from London to Tenerife is a course of 213.5 degrees, 23.1 returning, and from Tenerife to New York is 300.5 degrees, 85.8 returning.
If you take those three flight paths as the three sides of a triangle,
it has angles of 34.4 degrees at New York, 74.5 degrees at London,
and 82.6 degrees at Tenerife, which adds up to 191.5 degrees,
which is impossible for any triangle plotted on a flat plane.
Pilots and other people who plan the routes of aircraft do so by
calculations (nowadays generally done in software) that take into account the approximately spherical shape of
the Earth to do so (and nowadays even take into account the few miles'
difference between the diameter of the Earth measured from pole to pole
and the diameter measured across the equator).

I think that the aircraft should fly straight, not following the Earth curvature if nothing is done. This means, it should have a slight tendency to raise up but this may be only noticeable when it flies half way around the globe.
This effect may be much smaller than other factors that would alter the altitude. Anyway, if the aircraft is set to fly at some fixed altitude, it would compensate for the Earth curvature as well without doing anything special.

You are failing to realize the physics in the change of the angle of the vector for gravity as you move around the Earth.  You are also failing to account for the change in lift that you would see as the pressure changes for the given power setting you are using.  Both of these work to self regulate the plane.

The gravity vector will, indeed, rotate about the center of the planet as the body moves, and the lift would be reduced at the high altitude. But as autopilot is set to maintain the given altitude, it will probably adjust before these other two effects become observable.

agreed, but you started out with "I think that the aircraft should fly straight, not following the Earth curvature if nothing is done."  That's not true.  I absolutely agree that it's a super minor factor, but the first paragraph of the answer is incorrect.

Assuming we are flying "on the front side of the power curve"--
For a given power setting, there is only one angle-of-attack that will allow a given aircraft to maintain a constant altitude.
For a given angle-of-attack, there is only one power setting that will allow a given aircraft to maintain a constant altitude.
So your question is essentially, "is the required angle-of-attack, or power setting, different than it would be if the earth were flat?"
The answer is "yes-- very very slightly."
If your question is also meant to ask "is this something that a pilot must consciously take into account?", the answer is certainly "no".  Having a few pounds of dust accumulate in the nooks and crannies of the aircraft would probably make more difference to the required angle-of-attack or power setting than the fact that the earth is round rather than flat.
Likewise for effects due to the spin of the earth and its surrounding atmosphere.
If your question is also meant to ask whether an ARHS reference system or other similar system must specifically take into account the curvature of the earth to remember which way is "level" on a flight covering a long distance or whether this is automatically corrected for by other built-in functions of the system-- that is a much more complicated question but the answer is generally the latter.  The same is generally true about remembering which way is "level" over a long duration of time, as the earth rotates about its axis.
For self-contained inertial navigation systems that operate completely independently of any GPS-derived position information, the answer is quite different: in this case effects related to the spherical shape of the earth, and effects related to its rotation about its axis, are very important.


text:
What is the minimum radio and navigation equipment for required for VFR and IFR certified aircraft?
As per ICAO or FAA, what are the minimum radio and navigation equipment must be fitted in VFR and IFR certified aircraft and helicopters?

Welcome to aviation.SE

In the US, this in in 14 CFR 91.205, Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.
There's a lot of detail so you can check the whole thing, but looking only at communication and navigation items, the required equipment is:
VFR (including night VFR)

Magentic direction indicator
Altimeter (for airspace navigation)

IFR

Instruments and equipment as for VFR
Two-way radio communication and navigation equipment suitable for the route to be flown
Rate of turn indicator (for timed turns)
Directional gyro
A clock (this question explains why a clock is needed for IFR navigation)

That list might seem very short, but remember that under VFR there's no requirement to have radios and there are even some vintage aircraft flying around with no electrical system.
Under IFR it's fairly obvious that you need a radio for ATC communication and whatever navigation equipment is needed for your planned flight, e.g. you can't fly a GPS approach unless you have an IFR-certified GPS. But there are multiple approach types and they all have different equipment requirements so the regulations just give a general statement.
Note that commercial flights operating under parts 135 or 121 will have additional requirements; the information above is really the bare minimum that's legally possible for GA flights.

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Can foreign pilots get an FAA ATPL and work for a US-based commercial airline?
Could a non-US citizen come to the US, receive flight training, earn an FAA CPL and be hired by a US-flagged airline? Are there any specific rules regarding the employment of foreigners as pilots for a US-based commercial airline? For the sake of example, an Iranian flying for a US-flagged carrier?

Hi Moji, welcome to Aviation.SE.  Great question!  I reworded it to fit what I think you're asking - let me know if this is not what you intended.

also, this question may be relevant if you are seeking flight training as a foreigner.

Is the question, "Could a non-US citizen come to the US, receive flight training, earn an FAA CPL & be hired by a US-flagged airline?" or is it "Can a non-US citizen, holding a CPL from a foreign country, come to the US & fly for a US-flagged airline?" I see those as distinctly different questions, though I might be splitting hairs...

This question covers obtaining an FAA CPL.

I appreciate your effort to reword my question. I would love to know more about " could a non-US citizen come to the US, receive flight training, earn an FAA CPL & be hired by a US-flagged airline"?

Just to clarify, foreign or not, you need an ATPL (not just a CPL) to fly for an airline. While many countries do allow flying as a first officer on an airline without an ATPL, the U.S. does not (not on a U.S.-flagged airline, at least.)

@CGCampbell if path of citizenship makes a difference to the faa, i'd sue them for discrimination.  naturalized citizens have exactly the same rights as those born in the united states... except they can't be president.

I have deleted my comment and retracted my vote-to-close as unclear... as re-written, in my opinion this is now on-topic

At the most basic level, the DHS/Dept.Of State regulations regarding work authorization have to be followed. There are essentially three categories you can be in, as far as employment is concerned:

A naturalized or natural-born U.S. Citizen. You're allowed to work for any U.S. employer.
An (in-status) permanent resident (green card holder). You're allowed to work for any U.S. employer.
A non-resident. You're allowed to work only with DHS authorization.

The first two are somewhat self-explanatory. Some positions may have special requirements - for example, you can't be a U.S. President without being natural-born. Some employers place limits on non-citizen employment due to various U.S. laws usually related to access to information of military nature or applicable to military needs. Thus, a lot of aerospace manufacturing is off-limits to non-U.S. citizens, simply because the company might also handle aerospace jobs and doesn't wish to sufficiently compartmentalize access. For example, most if not all SpaceX jobs are for U.S. citizens only. No such limitations apply to normal commercial transport pilots.
Permanent Resident Workers
Unless you're already a permanent resident, you may wonder how to become one. In most cases, someone else - a family member or an employer - must petition on your behalf, you can't apply yourself. There are also strict yearly numerical limits for most permanent resident visa categories. Citizen or permanent resident family members 21 years of age and older can apply for you, but unless you're petitioned for by a spouse, the waiting times are measured in years.
We can review the directory of Visa categories, immigrant, with an eye for transport crew employment. I will elaborate only on the categories that apply to a pilot who isn't eligible for another category (say you're not an Iraqi who has worked for U.S. Govt, etc.).

DV category - Diversity Visa - if you seriously think of working in the U.S., you should be applying every year. It's a lottery. Make sure you apply at the official .gov website; the entry is free.
E Category - Employer-Sponsored. The first and most obvious requirement is that the potential employer must be willing to spend the money (on the order of $10k) to petition on your behalf. If they can find employees already authorized to work, there's very little incentive to petition.

Assuming that your employer is willing to sponsor an E immigrant visa on your behalf, the next step is to obtain a labor certification from the Dept. of Labor. A Pilot would most likely fall under the third preference, assuming that a labor certification can be obtained. And here's the real clincher: given the never-ending supply of commercial transport pilots in the U.S., and a lack of any particular special requirement for the job (we aren't talking about test pilots!), there's a nil chance of the DOL certifying the position. The DOL certification process requires the employer to prove that there is an insufficient supply of qualified workers who are already authorized and qualified for the job. So that is, essentially, where it ends.
Non-Resident Workers
The third category is the most complex one. There are various kinds of work authorization available, under various programs. All of them have special requirements, thus a non-resident is by default ineligible for any work authorization, unless he/she fulfills the requirements. For example, students on F-1 status are authorized to perform on-campus work, and for certain off-campus jobs iff they amount to practical training.
The non-immigrant category (usually Letter-Number, with an optional Letter suffix, e.g. F-1) is indicated on the Visa, but remember that the Visa is merely a document that is necessary (but not sufficient) to enter the U.S. After you're in the U.S., you or your employer can adjust your immigration status, as long as you fulfill the requirements to do so.
Thus, we can review the directory of Visa categories, non-immigrant, with an eye for transport crew employment. I will elaborate only on the categories that apply to a pilot who isn't eligible for another category (say you don't work for NATO or UN, you're not a NAFTA signatory citizen, etc.).

D category - allows you to work as a pilot, but you must be employed by an international airline, and you can't stay over 29 days. Your employment must be bona-fide; it can't be a sham set up just so that you can mostly work as a domestic pilot.

And that is, unfortunately, "it". You may be eligible for work if you're a citizen of one of the countries that U.S. has special agreements with, usually based on humanitarian and similar reasons, but those are few.
As you can see, FAA regulations don't even enter anywhere in the picture yet. The DHS regulations are a primary concern here.

Wow. Very thorough, thanks!

Dear Kuba Ober thank you so much for your effort to write in specific all kind of Visa Available for foreigners. Unfortunately this is a big disappointing to find out how difficult is that for foreigners to travel to US and get their pilot training done and have an eye for transport crew employment which is a great job and a dream for me. I am not sure if I understand completely how this story will work if I manage to get my PR ( green Card )? Does that gonna work? will I be able to be hired by an Airline and work legally?

@Moji Once you have a green card, you're set! Since getting a green card is a long process, you may wish to finish your training sometime before you get the green card. Your immigration lawyer (and you do need one) should be able to give you an estimate of when your application will become current; see also the visa bulletin. Just to give you an idea: unmarried children of U.S. citizens who applied in 2007 will be getting their green cards only this year!!

I would note, though, that the list of places you can't work without being a citizens is actually quite long, especially in the aviation industry, which is quite security sensitive. Even my current job, which doesn't deal with any classified information, we still don't allow non-citizens due to government information export restrictions that would make it pretty much impossible for a non-citizen to work here without a lot of really nasty paperwork that is nearly impossible to get approved even if filed.

@reirab That's true, but once you are a permanent resident, you can be a citizen in about 5 years, so it's not a permanent ban, so to speak.

Long story short, yes.
First, can non-US citizens get an FAA ATPL? Plenty of non-US citizens - myself included - hold FAA pilot's licenses (although I'm not an ATP). This question has more details, but as long as you're legally in the US with an appropriate visa, and you have TSA approval for training, then you can get any FAA license you want (or can afford!).
Second, are there any rules or limitations on foreigners piloting for US airlines? This is mostly a question about employment law: it's possible to hold a valid FAA license but not be authorized to work in the US. So again, if you have a valid visa or other work authorization then in general you can be employed by a US airline. Pilots do need background checks but obviously it is possible; Delta's pilot recruitment page simply says:

Current passport or other travel documents enabling the bearer to
  freely exit and re-enter the U.S. (multiple reentry status) and be
  legally eligible to work in the U.S. (possess proper working
  documents).

There may be some special cases for security-sensitive operations, but aviation in general is fairly international and you can find 'foreigners' flying in national airlines all over the world.

I'd bold it: it's possible to hold a valid FAA license but not be authorized to work in the US

In this case how we can be sure that there is a way to hold PR ( green card ) and work in an airline in US?

@Moji I'm not sure what you're asking? If you have a green card then you're allowed to work in any company that will employ you. And as the Delta page shows, at least for them there is no requirement to be a US citizen, only to be authorized to work in the US. A green card is one way to have that authorization.

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text:
Why is the XB-70's Top speed only about 60mph higher than its cruise speed?
The maximum and cruise speeds listed for North American XB-70 Valkyrie are:

Maximum speed: Mach 3.1 (2,056 mph, 3,309 km/h)
Cruise speed: Mach 3.0 (2,000 mph, 3,200 km/h)

Why is there such a small difference?

Designed in the era just before anti-aircraft missiles, it was designed to fly high and fast. It was designed to fly at it's nominal maximum speed at all time, so as to remain uncatchable and mostly untargetable by radar. My guess is the fastest it ever flew was the max speed, and they used an average for the cruise. It's cruise would be at full throttle in any case.

@CGCampbell I think you are correct, the SR-71 was similar. Max 3.3, cruise 3.2, they get better when they fly faster but are limited by temperature.

Note that the situation here isn't at all unusual. For example, the Boeing 777 has a cruise speed of 560mph and a top speed of 590mph.

@DavidRicherby True, though that's for a different reason (supersonic flow over portions of the wings/control surfaces.) I'd hope that XB-70 and SR-71 were designed to not need to be concerned with such things.

Co-workers are former SR-71 pilots (actually most are now retired) and they are clear that the thrust headroom for the SR-71 was substantial. The book said 3.3 but there was enough thrust headroom for higher speed.  The limitations were heat, as is extensively mentioned elsewhere.  So it did not run full throttle, except perhaps in TO and climbs. I understand the XB-70 had similar heat issues.

From the excellent point in the comments, the XB-70 was designed for one thing; speed. It was, until the Blackbird, the fastest manned air-breathing aircraft. It was intended to outrun Soviet interceptors like the MiG-25 and fly higher than ground-based anti-aircraft weapons of the day, making it a possible first-strike weapon. As such, it wasn't going to loiter around at 80% throttle.
A second point is that since the B-70 project was cancelled for several reasons (cost projections, development of ICBMs and of high-altitude fast-tracking SAMs), the two prototypes were used primarily as testbeds to push the envelope of air-breathing craft. As such, data regarding their fuel efficiency curve was secondary to more basic problems of aircraft design for hypersonic aerodynamic flight.

Actually, the MiG-25 was developed as an answer to the threat of the B-70 and the SR-71. And it worked: The SR-71 never overflew Soviet territory.

Actaully, Oxcart, the A-12 preceded the XB-70 in time.  The A-12 is a variant of the SR-71.

actually the sr71 is a variant of the a12

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text:
Under VFR, can a pilot fly above a layer of clouds?

Image source
I often see charts like the one above showing VFR cloud clearance minima. Since the regs list a minimum distance above clouds obviously VFR flights are allowed to fly over clouds. However, it's not clear what kind of clouds one can fly over: only individual smaller clouds, a broken layer or a solid layer. In the US does a pilot under VFR have to maintain a visual reference to the surface?

They don't usually give reasons for the rules. I didn't realize at first that the cloud-separation rule was not because clouds are scary. It was because IFR planes could pop out of the cloud, and you and they could get a nasty surprise.

Paraglider's in the UK just need to remain clear of cloud and have sight of the surface. It doesn't say that sight has to be directly below you...

Yes, it's legal to operate under VFR without any visual reference to the surface. It's called operating VFR over-the-top, not to be confused with VFR-on-top which is an IFR clearance. Over-the-top is defined in 14 CFR 1.1:

Over-the-top means above the layer of clouds or other obscuring phenomena forming the ceiling.

There are some exceptions and restrictions, though (this list may not be complete):

Recreational pilots must have visual reference to the surface (14 CFR 61.101)
So must sport pilots (14 CFR 61.315)
So must student pilots (14 CFR 61.89)
If the pilot has a foreign-based private license then all restrictions on the foreign license apply, which could prevent VFR over-the-top (14 CFR 61.75)
Large, turbine or fractionally owned aircraft must be equipped as for IFR (14 CFR 91.507)
If flying for an airline or other operator, their OpSpecs must allow it

Ok, let's see if aviation terminology can get more confusing. VFR-on-top is not in fact VFR. And VFR-over-the-top is a completely different thing. Ugh. No wonder I was confused!

@TomMcW Yeah, the other problem with VFR-over-the-top is that you, unless you're equipped and rated for IFR, you have to be sure you'll have VMC in which to descend on the other side of those clouds.

Is VFR-over-the-top an ATC clearance or more just a description of flying VFR over clouds? When would you use the phrase?

@TomMcW It's a description, not a clearance. Many VFR flights aren't in contact with ATC anyway. As it happens, I heard a VFR pilot trying to get down through a cloud layer recently, he asked ATC if any other VFR flights had made it through and if the controller could ask them where they found a hole in the layer. If he didn't find a hole I hope he had plenty of fuel to get him back to somewhere less cloudy!

@Pondlife That's why I was wondering if it was legal to do. It doesn't sound very wise unless you're at least IFR equipped and rated. You could get stuck up there

@TomMcW Yes, it's a nice example of legal vs. safe. [This question}(http://aviation.stackexchange.com/q/1636/62) has some more information but I'd say you need have a plan B for sure.

its perfectly legal to ask the controller if there are any airports reporting VFR or ceilings above your altitude. Note that under IFR, when your destination is above minimums, after you've flown missed the controller will ask you your intentions. At that point, you would be doing the same thing, asking for airports reporting above minimums or VFR. Alternate airports are for lost comms.

How about in EASA world? How you land?

@Andrius That would be a great question to ask, I have no idea what the EASA regulations are on this. As for landing, you still need to be IFR to go through clouds. If you get to your destination and there's no hole in the clouds then you have to go somewhere else, or switch to IFR if you can.


text:
How to determine the aircraft tail number on a particular flight?
How can one determine the registration number (i.e. N123AA) of the aircraft used on a particular flight, having the date, airline, flight number and city pair to start with?

Might be better for Travel.SE?

date in the past or in the future?

@fooot  -- An abbreviated version of the answer to this question was posted on this thread and it was suggested that it would be worthwhile to make that its own question/answer for others here who are interested in this "how to."

@RalphJ Thanks, I see the comment now. I guess it's helpful here too.

The step-to-step guide:
1. Search for Flight Number:

Get hold of the flight number, such as LH123 or AA789 for instance. This is usually found on your ticket, baggage tag or some email.
Enter it on a site such as www.flightradar24.com or www.planefinder.net.
Note down the registration (such as N1234A or D-ABYT) for the flight on the day your are interested in.

2. Search for this Registration:

Try entering it on for instance www.planespotters.net which is often considered a reliable source for aircraft data.
You should be able to get the aircraft history:

First flight and delivery dates (and hence aircraft age)
Previous operators and registrations.
MSN (manufacturer serial number)

One good source for photos of a particular airliners is the Airliners.net Advanced Search option; enter the registration in the Registration search field, and you'll get back a surprising number of photos of that aircraft, usually taking off and landing, although sometimes photos of the flight deck as well.  Also, a Google search on the registration can often yield pictures and info as well.

The reg number's only set in stone once the plane takes off, so you'll only get this information for past flights. Equipment can and often does change between hops on the same flight number, and a multi-stop flight can even arrive at one gate and depart at a different one because of a scheduled plane change (or a last-minute one). The routing/scheduling center for the airline will know the expected equipment to be used, but that information doesn't technically become public until the flight plan for the hop is filed with the FAA, which for major airlines with pilots and ground staff coordinating each flight, can happen as late as the aircraft taxiing to the runway.
You can try http://www.flightradar24.com/. It's an aviation database of commercial flights updated in real time. Here are the most recent stats for Southwest flight WN3223, a trans-con multi-hop route from NY-Laguardia through Dallas Love, Phoenix, LAX, Oakland and Sea-Tac. There have been a few different tails used for this flight number over the past few days, including N7732A, N746SW and N260WN. This is common for Southwest, as they operate the B737 exclusively, and with very few exceptions any plane in the fleet can handle any route if need be (they did get some new -800s recently which have slightly higher passenger count so they're not completely interchangeable).
As you can see, information on tail numbers is not complete; it should be sufficient to assume the tail doesn't change between stops on the route unless a new one is specified. If you really have to know, the airline can probably tell you what equipment was used on a particular flight if you ask really nicely (or you have some input on possible maintenance issues), but they won't be able to guarantee a particular tail number for an upcoming flight unless the route is such that only one plane in their fleet could do it (which is rare and becoming rarer but still not unheard of).


text:
Do any ATC displays use colours to indicate altitude?
I've been thinking about how ATC displays present information to operators and it seems that colour isn't used, at least for critical information like altitude. Since our eyes can recognize colour more quickly than they can read numbers, using colour to display altitudes seems like it would be helpful for operators.
Do any existing ATC displays use colour to encode altitude?

I can't see any use for this.  At the 9 o'clock position, there are two yellow (medium altitude) aircraft heading towards each other.  Are they at the same altitude?  What do "high", "medium" and "low" mean? Here, I can see instantly.

I feel that more questions linking to your site would be more a way to promote it than to get answers to satisfy your curiosity.

Thinly conceiled spam.

@Simon that image should say "altitude in hundreds of feet", I've never seen a data block display thousands. Or that aircraft is at FL600. Which is cool, in it's own way. :)

@falstro 060 is 6,000 feet.  It's effectively a flight level.

@falstro Doh, yes, I just remembered :), but it looks right, 060 is 6,000 feet.

@Simon exactly, it's in hundreds, 60 * 100 = 6 000 ft. It states that it's in thousands of feet, which would be 60 * 1000 = 60 000 ft.

That isn't a "three-dimensional radar display". It is a two-dimensional display with altitude encoded as color.
Current ATC radar is two-dimensional, with altitude encoded as a number:

A true Three-Dimensional Radar would look something like this:

(Photo liberally thieved from a 1956 edition of Popular Electronics.)

As far as 2D radar goes, I don't believe color-coding the targets as described in the linked implementation would provide an improvement over the existing system: It's based on "bands of interest", which means you need to define what you're interested in, which means keystrokes - the controller is typing in what altitude they're working with to change the colors so they can see any conflicts, then they will need to change to the next altitude of interest, etc. - a lot of work versus "This number is within 10 of that number. There may be a conflict if these aircraft get too close to each other."
In addition color-coding the targets as described forecloses other possible uses of color in the system, and doesn't eliminate the need to display the altitude (controllers still need to know the precise altitude of an aircraft to determine how far off its assigned altitude it is -- in fact that may be a more apt use of color coding).

I guess in 1956 Popular Electronics didn't think much of Texas, huh?

@mins We're all living in the holodeck on the Enterprise!

liberally thieved - well played!

@mins We've been trying to give it back to Mexico for years, but they still remember the Alamo.


text:
Do airports use buried cables to detect planes or vehicles moving?
I've been thinking about the possibility of using a buried cable to detect aircraft or other vehicles moving on the ground at an airport. The military already uses this technology for other purposes and it would avoid some issues with radio- or radar-based solutions, like ground clutter.
Are buried-cable systems already in use for tracking surface movements at airports?

I don't understand exactly what you want to detect, hence difficult to answer. Movements and intrusion can be detected by so many ways, depending on the purpose. What is not already detected or not well enough?... Could you clarify your question by editing your post? Thanks.

See Also: http://aviation.stackexchange.com/questions/19087/is-there-any-system-to-warn-of-runway-incusion & http://aviation.stackexchange.com/questions/9960/what-is-the-approximate-accuracy-of-asde-x

Airport ground support vehicles, like the aircraft they support, are outfitted with transmitters that broadcast thier GPS position to ATC ground controllers (ADS-B). That infrastructure is already in place and implementation is ramping up rapidly.
I think that may solve the problem you are addressing unless you are thinking of a security issue.

Not to mention ground radar and a number of other systems.


text:
Does a 4 bladed prop have almost twice the thrust of a 2 bladed prop?
Does a 4 bladed prop have almost twice the thrust of a 2 bladed prop?
As a general rule, I'm told GA planes generate 4 lbs of thrust per horsepower. (Source: Zenith School of Airplane Design, Flying on your own Wings)
So if you use a 4 bladed prop, does it generate anything close to 8 lbs of thrust per hp?  In other words, say for an ultralight, where you're only flying 60 mph, and prop drag isn't an issue, can you use an engine with half the horsepower, or some other fraction thereof?
Will that shorten my takeoff roll? Conceptually, takeoff roll is just the distance to accelerate from zero to my minimum takeoff speed.
Since F=ma, then a=F/m, so if I want to accelerate twice as fast, I need twice the force, or thrust ( probably more than twice the thrust, to overcome rolling drag, etc.)
Is this correct, at least conceptually?
Just to summarize:
In the GA world, I'm told as a general rule, props develop 4 lbs of thrust per hp. So, for 160 hp, I get 640 lbs of thrust, or is this not correct?
In the STOL world, with 8' props, I'm told, they get 8 lbs of thrust per hp. So, for 160 hp, I get 1,280 lbs of thrust.
Well, I'd rather have a 160 HP engine and prop that produces 1,280 lbs of thrust than 640lbs of thrust.
So... if I take my 8 ft 2 bladed prop, chop it in half to get a 4' 4 bladed prop ( same overall "wing area"), I'll still get 1,280 lbs of thrust ( less a few % for inefficiency), instead of 640 lbs of thrust. Is this correct? If not, what value of thrust should I expect, roughly?
Or are these rough estimates incorrect?

Related: https://aviation.stackexchange.com/questions/37720/how-does-blade-solidity-ratio-relate-to-thrust-power-torque-of-a-propeller

At the same size and rpm, a 4-bladed prop will require twice the horsepower to drive, approximately.
Due to blade interference, it will always generate less than twice the thrust. So lbs thrust per hp will generally slightly decrease, as long as the propeller is in its optimal tip airspeed band.
If the propeller was overly fast initially, slowing it down and going with more blades to compensate can result in small thrust/hp gains (+X percent, not X times). This will generally apply to aircraft originally designed with much less power and later modified.
There are a few legitimate cases where design size is less than the aerodynamically optimal size. One is clearance to the ground or between the props (in quads). Another is turbofans: the drag of the nacelle overcomes the benefits of lower disc loading. Even for airliners there's a push for wider fans, but the "open fan" idea (basically a turboprop) was scaled down to geared fans and the Ultrafan for noise reasons.
In helicopters, at some point, the blades become so long and so far apart that adding more blades beats the weight and bulk of longer blades. The more blades, the worse, so going up to 4 is not such a big deal as doubling it again to 8. For small aircraft with low power/weight ratios, where 2 blades fit easily, that's generally the best configuration.

I guess so in theory.  That assumes the prop was optimized in the first place, but in the GA world, I see ground clearance limiting prop diameter in many cases. Case in point, many float pilots put on a larger prop once they are on floats, my soaring club wanted better climb performance from their tow plane, so they bolted on a 2nd prop, offset 90deg  to the first one. Lazair did the same thing, all without increasing horsepower. I'd say there is probably at least a 20-30+% improvement, or else no one would bother doing it. Of course cruising speed would suffer.

This is probably the case of a prop optimized for cruise at full course pitch where you need full power. I'm thinking they couldn't get their prop pitch fine enough and there was extra power to be used.

@Fred If you're diameter limited, then optimization may be achieved at more than 2 blades. This is why large airliners can have 40 blades in their fan.

Will blade interference not make it require less than twice the horsepower to drive?

@rackandboneman Can require a little bit less due to the spin. But very nearly twice.

Blade interference makes it so that more horsepower is required for more blades. More blade profile resistance to swirl around.

@Fred: Yes, size restriction is generally the only reason people increase number of blades. Same is true for helicopters but usually the size restriction there is structural strength (strength is the same for airplane wings - also the reason for biplanes: increasing the "blades" of wings)

Does this suggest that a single blade with a counterweight would theoretically be better than two?

@Reid Yes, in terms of aerodynamics only. Overall it will be less efficient, as the counterweight is wasted weight. Some gliders do use single-blade propellers, but only for the ease of stowage.

This is not conceptually correct, for the following reason: Doubling the number of blades on your prop will not double the thrust unless your engine is powerful enough to drive the prop at the same speed as the 2-bladed case.
For small planes with fixed-pitch props, a prop and engine combination is chosen for a given aircraft so that when the engine is running at its maximum RPM setting and full throttle, the prop is absorbing the full power output of the engine. If one then adds some more blades to the prop, the engine will be loaded down too heavily for it to run at its maximum power setting and the power output of the engine will go down and so will the thrust generated by the prop.

As I read this there are really two questions being asked here.
1) Does a 4 bladed prop have almost twice the thrust of a 2 bladed prop?
Basically yes, if the propeller rotation rate, blade length, inflow velocity, and blade pitch all remain the same.  There will be some losses for interference between blades, but those can be neglected to first order.
Going from 2 blades to 4 will also, those same things being equal, double the required torque, which will also double the shaft power required to spin the prop at that same RPM.  If you double your number of propeller blades AND double the size of the engine, you will nearly double your thrust.  This should significantly reduce the takeoff roll.
2) Can I add blades to my propeller to compensate or allow for a reduction in engine power?
The answer to this is a more nuanced no.
At the most basic level, the useful flow power of a propeller $P_{flow} = TV_\infty = P_\mathrm{shaft}\eta_{p}$
Restated, $T = \frac{P_\mathrm{shaft}\eta_{p}}{V_\infty}$
This shows that, for a given amount of engine power at some airspeed, the useful thrust you can get is driven by your propeller efficiency at that flight condition.
In general, a well-designed 4-bladed propeller is about as efficient as a well-designed 2-bladed propeller.  More blades is desirable when you need more thrust in a small diameter, less blades gives you a slight efficiency benefit because of the reduced losses from blade-blade interactions. But one won't give 2x the efficiency of the other.
Prop efficiency varies widely, depending on the operating condition and design details.  So a specific 4-bladed propeller might very well give more thrust than a 2-bladed propeller on the same engine, depending on how well-matched it is to the engine and the design details of the propeller.  The converse may also be true.
QPROP is a good, open-source tool for playing around with the effect of changing the number of propellers blades, operating condition, and other design details. The theory document also lays out the fundamental relationships clearly.

There is no free lunch here. Adding blades does not somehow cause the engine to produce more power. It just means that each blade gets a smaller fraction of that power and thus produces less thrust, and total thrust would remain constant.
Actually, due to interference, total thrust actually goes down a bit when you add blades. The main reason to accept that inefficiency is that any fewer couldn't absorb all the power coming from the engine, typically due to ground clearance or tip speed, so you don't really have a choice.

To my understanding, one single blade would be most efficient. However, for obvious balancing issues, it is never used.

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How to calculate minimum flying speed in distributed propulsion?
How to calculate minimum takeoff speed due to distributed propulsion?
Is there a quick way or rule of thumb way to calculate effects of distributed propulsion?
I understand NASA is doing research in this area and is determined to prove an increase in efficiency of 500% in real world tests, although this may include the benefits of electric distributed propulsion.
I understand they swapped the wings out on their Dornier, from 17psf to a 45psf wing loading.  That's an increase of almost 300% in just the aerodynamics!!
So....
I understand the bigger the prop disc, the more efficient the prop.
if I have a small ultralight with 25 hp, and now use two 12.5 hp motors instead with newly optimized props....
What is the approximate % increase in efficiency/thrust as the prop disc is now twice as big? ( assume only a small weight increase)
Now let's say I have a 10' span, with 2 motors with 6' props. The entire wing and tail is in the prop wash.
What is the formula for propeller exit speed(v2)?
Is this my new indicated airspeed(v+v2)?
In other words, if my stall speed is 40mph and my prop exit speed is 20 mph at zero ground speed distributed over the entire wing, can I take off at a 20mph ground speed? i.e IAS= 20mph ground speed + 20 mph prop exit speed= 40mph need for takeoff.
Is this correct, at least conceptually, after all, it's just like a headwind, no?

You seem to have an unrelated question about prop efficiency buried in there. May I suggest you take the site tour: https://aviation.stackexchange.com/tour ? You may also have missed that there is an option to accept answers, which the people who have answered your 30 questions so far would likely appreciate :)

Some research would help. For instance, start with: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980232082.pdf

The better way to look at predicting the performance of a distributed propulsion wing (specifically the NASA X-57 wing concept) is to think of the propellers along the span as a high-lift device that augments the lift of the wing through interaction with the trailing-edge flaps, rather than as headwind-generators. In other words, the propellers increase wing C_L, allowing you to have a smaller wing for the same landing and takeoff distances.
With many small propellers, you get performance that starts to resemble a jet flap or externally blown flap, but without the complexity of internal ducting or non-uniformity due to a small number of engines.
Optimistic rule of thumb from thin airfoil theory is that, for a 2D section
$c_\ell = \frac{\partial c_\ell}{\partial\alpha}\alpha + \frac{\partial c_\ell}{\partial\delta_F}\delta_F$
$\frac{\partial c_\ell}{\partial\alpha} = 2\pi(1+0.151\sqrt{c_J}+0.219c_J)$
$\frac{\partial c_\ell}{\partial\delta_F} = 2\sqrt{\pi c_J}\left(1+0.151\sqrt{c_J}+0.139c_J\right)^{\frac{1}{2}}$
where $c_J = 2\frac{h_J}{c}\frac{V_J}{V_\infty}$
$V_J$ can be calculated from propeller momentum theory based on disk area, input power, and assumed efficiency.
Bear in mind this is for section $c_L$, and will need 3D corrections for the whole vehicle based on percentage of span with propellers, induced flow, etc. It is also reasonable only in the limit of small flap deflections and angles of attack. Wind tunnel data or flight test reports might be a better place to get a first-pass estimate.
https://www.researchgate.net/publication/299641308_Comparison_of_CFD_and_Experimental_Results_of_the_LEAPTech_Distributed_Electric_Propulsion_Blown_Wing
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740003719.pdf
https://arc.aiaa.org/doi/abs/10.2514/6.2019-3170

You have great content and great links.  A helpful way to combine to two is by using the hyperlinks as references in line with the content.

If NASA swapped out the wings of their test plane, from a wing loading of 17psf to 45 psf, that's a factor of 2.6. I understand their test plane does not leading edge slats, but does have slotted flaps, so max cL is about 50% x 1.5 plus 50% 2.8 = max cL of about 2.  What that tells me is that the new wings generate 2.6 x 2 = 5. So... distributed propulsion can raise the cL from 2 to 5. Is it really that much?!?!?!?

Yes, maximum predicted CL of the X-57 is in the 4-5 range.  But in principle with sufficient power it's possible to generate much higher values, up to 10 or more.


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Need to add elevator downforce to total lift when sizing wing area?
Do I need to add elevator downforce to total lift when sizing wing area?
Say a small airplane weighs 1,000 lbs, and both the wing and elevator have a max Cl of 1.5, and my elevator is 20% of my wing area, so max elevator downforce is roughly 20% of 1,000 lbs or 200 lbs in a classic configuration.
Doesn't the main wing then have to carry 1,000lbs + 200 lbs negative elevator
lift, for a total lift of 1,200 lbs?
If the plane is in a tandem configuration, the main wing would only have to carry 800 lbs, as the elevator has 200 lbs of positive lift,for a total lift of 1,000 lbs.
1,200/800 lbs is 150%!!, so if I use the tandem configuration, my main wing can be 50% smaller!!! Is this correct?

Have you thought of any other cases where your wing would need to provide more lift than the weight of the aircraft? Like during a turn?

Generally Eva,hated you are right, but if the front wing is underneath the fuselage it will be functional nearly over all its surface, but the other one is at mid fuselage thickness or above the fuselage so the wing area lift near the fuselage need to be more precisely evaluated.

This question could be improved -- you are really asking about the downforce created by the horizontal stab and elevator together, not just the elevator.  Likewise re area.  You seem to be a little unclear on the meaning of the word "elevator".

No.
First, your horizontal tail will need some control margin. A trimmed state where it flies close to c$_{l_{min}}$ is a grave design error. Next, your horizontal tail sees the same angle of attack change as the wing, reduced by downwash. This means that the strongest downforce is trimmed in fast flight, when the angle of attack is small. Fly slower and angle of attack rises not only on the wing, but also on the tail. Chances are that your tail produces positive lift in slow flight.
Flaps will change this picture slightly. Setting flaps will need you to re-trim the aircraft because now the center of the wing's lift has moved slightly backwards, so the load on the tail becomes more negative.
Fun fact: If you have a symmetrical wing airfoil, the trimmed tail load will be constant over the whole envelope. In order to achieve the best control power, this tail load should ideally be zero.
Now for the tandem wing: You assume that now both surfaces create lift, so overall drag should be lowest. But adding area on the tail will reduce the needed wing area. Given that the planform shape stays constant, less area means less wing span. However, induced drag is proportional to the span loading, and a lower span will mean more induced drag. Therefore, a conventional design with an unloaded tail of minimum size will have the lowest drag.

I think it's possible that this answer could be improved. Yes at first glance it seems obvious based on simple geometry that the tail must make the strongest downforce during high-speed flight, but this line of argument doesn't consider the effect of the pilot's pitch control inputs which change the position of the elevator or stabilator in order to trim for the desired airspeed.

Perhaps a better line of argument would be based on the wing's pitching moment coefficient-- which in fact is exactly the argument you offered for the case of the symmetrical (wing) airfoil.  You could then note that a cambered (wing) airfoil will generate a stronger nose-down pitch torque at high airspeed than at low airspeed, so the tail must generate more downforce (or less upforce) at high speed than at low speed, at least in the no-flaps case where the wing airfoil is held constant.

Your description would apply to a situation where we have an all-moving horizontal stabilizer (simpler to consider than the elevator case) with an airfoil the same as the wing but upside-down, flying at a negative (downlifting) angle-of-attack that is exactly equal and opposite to the wing's positive (uplifting) angle-of-attack.  In practice you would never have a horizontal stabilizer flying at this strong a negative angle-of-attack and creating this much downforce.  But yes, if the CG were so far forward that this much downforce were needed for balance, the situation would be as you describe, and this would definitely add a great deal to the total lift that the wing would have to generate.
So the basic answer to your question is "yes", but you have chose a very extreme case of a downlifting tail for your illustration.   In practice it can be shown that a configuration with a slightly-downlifting tail is generally more efficient than a configuration with a strongly uplifting tail.

("chosen" not "chose")

You can use the edit button to fix typos.


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Cp, Cg and Np location of a tandem wing configuration, both have positive lift?
I understand that in a tandem layout( 1 wing at front of plane, 1 wing at rear, both have positive lift) , the Cp has to be ahead of the Cg, and the Np behind the Cg.
Is there a rule of thumb how far forward the Cp has to be in front of Cg?, like 17% to 33% of MAC, etc.
Should Cm vs alpha, still have a negative slope?
Is it possible to have a 50% lift split between rear and front wings, just have a higher incidence in front wing, so it stalls first.
Any help would be appreciated

Cp of the forward wing will be ahead, Cp of the read wing will be behind and overall Cp will coincide with Cg, otherwise the aircraft will pitch up or down (because Cp is, by definition, the point around which the pitch moment is zero—if it does not coincide with Cg, then the net moment is non-zero and the aircraft accelerates in pitch).

I understand that in a tandem layout, the Cp has to be ahead of the
  Cg, and the Np behind the Cg.

No. Yes.
For a true tandem (without horizontal tail), it makes little sense to talk about an isolated wing. The combined CP will always coincide with CG (in a static flight). Otherwise, as Jan already noted, the airplane will pitch up.
For NP (neutral point), see below.

Is there a rule of thumb how far forward the Cp has to be in front of Cg?, like 17% to 33% of MAC, etc.

Forget about CP. It's useful in aerodynamics (and somewhat in structural design), but for the dynamics and stability analysis only NP matters. CP is very inconvenient, it moves all the time.
All these 'rules of thumb' apply to NP, not CP. In particular, the distance from CG to NP (CG being ahead) in terms of MAC is your 'static stability margin', and is the main contributor to static stability.
The rule is roughly the same regardless of the aerodynamic configuration. Some minor effects will be different (for example, damping will probably be lower), which may require some minor adjustments.

Should Cm vs alpha, still have a negative slope?

For static stability, yes. This is essentially the definition of static stability; how you achieve this slope is another matter.

Is it possible to have a 50% lift split between rear and front wings, just have a higher incidence in front wing, so it stalls first.

Yes. You'll normally need a somewhat smaller front wing for that. Or you can have a swept front wing and straight rear. Note that if you want a stable aircraft, at a higher angle of attack by definition the lift on the rear wing must rise more than on the front one. If you want to keep this AoA (e.g. at a lower speed), you'll need to dump this extra gain via re-trimming.

The lift curves should "cross" before stall at higher AOA, at lower AOA you want more lift in front to "pull up".  This all of course remains dependent on torque around CG as well.

"[...] simply by pitching the fore wing to a higher AOA so it stalls first" And what will happen when the aircraft pitches down and the front surface finds itself less loaded than the aft one? Sounds like a poor solution.

Start with a "conventional" plane.  Make the Hstab flat and as large as you like.  Set the decalage to optimal wing AOA.  With proper CG position the plane flies entirely on its fore wing.  If weight is moved back to make a "tandem", then rear wing must generate lift.  But the fore wing must stall first.  One can progress all the way to XB-70, or stop at tandem.

The downwash from the front wing already ensures a lower lift curve slope on the rear wing, and sweeping the front wing will make it stall at higher angles of attack - not really what you want. the lift on the rear wing must rise more than on the front one please add "relative" to lift. Otherwise this would imply a higher lift curve slope on the rear wing (by using flaps?) which will not help in terms of stability.

@AEhere, it's a bad wording on Robert's part—he apparently meant by setting it at higher angle of incidence.

OK, Peter mentioned something interesting (no, not keeping tail near downwash) about changing the geometry of the tail. All kinds of things come out of the wing in slow flight, could the tail "swing" or increase camber as well?  Just seems a shame to make it too complicated.  Imagine what could be flying with 5 mile long runways.

@Peter, this paragraph is in the context of 50-50% lift distribution as a goal. If we don't want a smaller front wing (just as a theoretical exercise), we can make a swept wing. Or something else. A very strong dihedral perhaps. It may not be the best, but it will fly; stalling the front first is just a safety measure and not a prerequisite. The same for lift changes. We can have a higher slope on the rear wing (why not?), and we can reduce downwash (as Rutan presumably tried on Quickie).

You get a higher lift torque slope in rear as plane pitches up with greater AREA of lower AR rear wing. This is simple weather vaning.  So, a higher AR straight in front, set at a higher incidence (thanks Jan), relative to the rear wing, will have higher lift torque at lower (shall we call it fuselage AOA?)AOA, but as F-AOA increases, weather vaning (bottom lift from rear wing) will increase as a faster rate and the torque curves will cross.  Similar to conventional tail.


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Is lower wing loading safer?
Is lower wing loading safer?
Looking at the ultralight specs in Canada, they call for a max wing loading of 12psf.  In fact, amateur built aircraft in Canada cannot have a wing loading over 20psf,I'm told.
While in the US, we see aircraft like Rutan's Boomerang at about 42psf.
Seems many GA aircraft seem to be stuck at 14-17psf range
With VG's or slats and flaperons  (full length flaps) and then slatted
(junkers flap(?)), Zenith is getting a nice cl of about 3.0, about double a typical ultralight
Why not higher wing loading?  This would lead to smaller wings, better aspect ratios, less drag, which means smaller engines, which means more affordable GA aircraft.
In fact, Rutan's motorglider (solitaire(?)), had a top speed of about 200(?) mph, with just a 4 cycle 18hp industrial engine ( rated for continuous duty), the kind you can buy at a big box store for $500.
I understand M. Coulumban, the designer of the cri-cri, just switched to low cost industrial honda engines as well.
On a saturday morning 1,000 mile cross-country trip ( pretty routine for the long-eze crowd)and have engine problems? No problem. Just go to a big box store and buy a new one and install it in an hour. Might as well make it an inline twin for that low of an engine price. Rutan's twin Defiant could take off from either engine alone, or both.  Seem's pretty foolproof to me.
Seems like pretty good economics for GA aircraft in my mind.
So..... why limit ultralights to 12psf.  I assume lower wing loading means it's safer.  Lot's of wing means it's harder to stall.
Is this correct?

Per Wikipedia, Solitair used Cuyuna 215 "The 2si 215 is a family of single-cylinder, fan-cooled, two-stroke, single ignition, aircraft engines that were designed for ultralight aircraft."  I think you may be overstating the engine availability at Home Depot. All their engines are listed as "Ship to store for pickup", so you'll grounded for some number of days waiting on that. Removing gas tank and other configuration issues might add to the swap out time, and most seem to be 4 stroke engines also, and I suspect may need unleaded gas.  Have to wonder if oil reservoir is large enough also.

I understand the original Solitaire used a 18 hp Onan industrial engine designed for water pumps. In Canada we have a big box store called Princess Auto.  They must have 20 gas engines just sitting on the shelf.  I understand these engines are rated continuous duty at 75% rpm, so oil and heat shouldn't be a problem. Yes, they use unleaded gas but that's cheaper anyways.  Rotax 912 engines are designed for unleaded gas, but you can use leaded gas in a pinch. Just reduces your TBO.

Wikipedia says "The initial engine was the 22 hp (16 kW) Zenoah G-25, but this was changed to a Robin engine and then later the 20 hp (15 kW) Cuyuna 215. The KFM 107e 22 hp (16 kW) engine has also been used."  No mention of Onan.

A book on Rutan's designs called Race to Space(?) mentions the Onan. It was a kit plane. I'm sure people used all sorts of engines.

@Fred You're thinking of the original tandem wing Quickie with the Onan.  Solitaire was a motorglider using a folding engine.  Solitaire was a flop because it was found that the canard config did not lend itself to flying close to stall with a decently low sink rate and it wasn't very good at thermalling.  In a normal glider you thermal with the main wing on the edge of the stall (and often the lowest sink rate is with buffet onset) and canard by design doesn't allow for this.  Anyway, the Onan was heavy for its power.  In Europe the 2 cyl BMW with Rotax gearbox conversion is popular.

Yes in general it's accurate to say that lower wing loading is safer, especially in the hands of a pilot with low experience flying in calm weather with an unreliable engine.  Flying in gusty winds and strong turbulence, there would be safety advantages to a higher wing loading.
The basic idea of limiting the wing loading is to limit the stall speed and thus the destructive energy involved in a forced landing.  As the question suggests, perhaps it would be wiser to limit the stall speed, or to allow deviations from the prescribed minimum wing loading in the case of a demonstrated low stall speed.
However the Canadian regs may also be written with the idea of limiting the top speed too.  Exactly why that's a good thing is not completely clear, but in the US at least an ultralight must not be able to exceed 55 knots in sustained level flight.

Future edit- "perhaps it would be wiser to actually limit the stall speed"


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Will a self balancing wing using an Eppler 186 airfoil still generate a Clmax of about 3+ with slats and slotted flaps?
Will a self balancing wing using an Eppler 186 airfoil still generate a cl of about 3+ with slats and slotted flaps?
Although the NACA2415 used in the pegasus wing for the Zenith 701 has a low Cm, the Eppler 186 has an almost zero Cm at an AOA of about 3 deg.
Just wondering if there is some aerodynamic reason why it couldn't generate a Clmax of about 3, the same as the NACA2415?
Of course, it would only be self balancing in cruise, without the slats and flaps deployed.
I understand not all airfoils see the same increase in Clmax, due to the leading edge, etc.

Do you mean, can slats and slotted flaps when deployed not overly affect Cm?

Are you designing a flying wing? Otherwise, why would you be using a reflex airfoil for your main wing?

@Jimmy Perhaps to minimize/eliminate trim drag?  He'll need an empennage when the flaps and slats are out, but in cruise, that could be trimmed to zero lift, zero alpha and produce only its form drag (which can be pretty danged small).

My design is a biplane with a 10' span with folding wings and a 10' Lv. I understand the Lazair had a very short Lv and used a double reflex wing.  So I'm trying to minimize my horizontal tail area by using a reflex wing as my Lv is so short. Think Bumblebee. Is this thinking correct? Works pretty good in XFLR5!!

@ZeissIkon Trim drag may be small potato compared to the efficiency loss by choosing a reflex airfoil. The CLmax loss alone decreases performance. And you can only get zero-lift on tail for a single design point at a single speed.

@Jimmy Yes, a single point, which would be your cruise speed which is about 90% of the time, no?

If you stagger your upper vs. lower wing, you can use relative incidence to do what that reflex airfoil would, and (I think) with less loss of efficiency.  You'll probably still need a pitch surface when your flaps/slats are out, though.

@Fred I don't know your particular design objective, but most of the time no. Unless you have a single weight, CG, speed, altitude you'll be flying all the time.


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Why is the Piper PA-23 considered a bush-plane, while the Beech 95 Travel Air is not?
The Beech 95 Travel Air was developed as a direct competitor with the Piper PA-23. The Piper is considered a reasonably ok bush plane, yet the Beech 95 is not commonly seen as a bush plane (I don't think I've ever seen a Beech 95 on floats).
What difference makes that the one is suitable as a bush plane, while the other is not?

Beech 95 Travel Air

Piper PA-23

Without getting into the details of the two, there are going to be 3 factors:

Landing Gear suitable for rough fields.
Decent size cargo loading doors and cabin with cargo provisions.
Power to weight ratio suitable for short rough fields.

Neither of those airplanes are really purpose-built for bush operations, but to the extent they have those characteristics they could be said to be more or less suitable.
When the US Army was looking at the DeHavilland Canada Beaver, which WAS specifically designed for bush operations, being designed in response to an operator survey, there was a hue and cry from the US aviation lobby, prompting the Army to have a fly-off with the Beaver vs a motley assortment of "utility" aircraft put forward in haste by various manufacturers (including the Travel Air IIRC, and the Cessna 195).  The Beaver won without much difficulty, and the US Army went on to take nearly 2/3rds of Beaver production.

Both the question, and this answer, are largely opinion based.
While the PA-23 was used more in the bush, neither aircraft can really be considered a "Bush Plane". The PA-23 designation includes both the Apache and Aztec, so any answer should really include both the B95 Travel Air and B95-55 Baron.
The Apache and Travel Air had similar performance capabilities, and so did the Aztec and Baron, so that is not a factor. Maintenance and reliability can be factors, but difficult to quantify.
Piper built 2047 PA-23 Apaches and 4930 PA-23 Aztecs, while Beechcraft built only 720 Travel Airs and 3,651 Barons.
So the popularity of the PA-23 in general, and "in the bush" probably has a lot to do with the fact it was put into production first, and was also cheaper to buy.

It seems odd to me that a low wing design would be used for landing on unimproved ground

Unimproved ground means, at worst, gravel or mowed grass for those airplanes.  You wouldn't try to run those things off of an unprepared surface.  The tires are too small and the gear not really made for it.

Setting aside how do these two planes fly, it is enough to see how do they stand on the ground. It does not look possible to drive over meadows with a car that has bottom that low and the wheels that tiny, so how could you land, take off? Also, the propeller is so low that seems ready to dig into the first mole hole on the way. I think they both clearly require some prepared strip.
See the Beaver for comparison below (source). I think these two aircraft in the question are not a real bush planes, none of them.

This is right at the edge of recollection for me, but my memories of Australian, Pacific & SE Asian general aviation in the 60s & 70s are that the Apache was largely reviled as underpowered & not suitable for even slightly marginal conditions or use. It was never spoken of highly by pilots.
The Beechcraft (in the above markets) was almost certainly more expensive to buy, more expensive to maintain, somewhat fragile, & considered a nicer, more up-market aircraft. I remember them as having a much more luxurious interior to Pipers & Cessnas. It's possible they were better soundproofed.
The Aztecs, on the other hand, were everywhere. Government departments, flying schools, charter operations, 3rd level transport, aerial survey, mail delivery, inter-island services, you name it.
They were sturdy & sufficiently well-powered enough to deal with short, unpleasant airstrips regularly, while carrying maximum loads. They were reliable, & usually well-liked by pilots & operators.
This niche was (sort of) shared with the slightly larger mid-sized 4xx Cessna twins & Britten Norman Islanders, but I recall few Barons.


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In the B737NG, is the CTR tank fuel the first to be pumped when there is fuel in all tanks?
In a B737NG, assuming you have fuel in all tanks, Wings and Center, and that all pump switches are ON, will the fuel from the CTR tank be the first to be pumped, despite all Fuel pump switches being in the ON position?
Will this happen even if the CTR fuel pumps are OFF and only the Wing Fuel pumps are ON?
And why are there so many warning in the FCOM about pumping the CTR tanks and warnings about not doing so in climb or descent if under 1000 lbs of fuel in the tank?
I have already read the "Why are fuel tanks in the wings filled first, and why are they used last?" post and answers.

By the time that's happening to the pumps in the wing tanks, you've run out of fuel, so there is a bigger problem than an uncovered pump at hand.

@StamatisVellis Mainly it's that you never get the wing tank quantity that low in the first place.

Because the center pumps effectively put out more pressure, the fuel from that tank will be used first when all pumps are on.